Researchers at Institute of Complex Systems from the University of Barcelona and Paul Scherrer Institute PSI have managed to explain the strange behaviour of microgels. Their measurements using neutron beams have pushed this measuring technique to its limits. The results open up opportunities for new applications in materials and pharmaceutical research.

This kind of fluids flow through our arteries, add colour to our walls or make milk tasty: tiny particles or droplets that are very finely distributed in a solvent. Together they form a colloid. Whereas the physics of colloids involving hard particles – such as colour pigments in emulsion paint – is understood well, colloids involving soft particles – such as haemoglobin, the red pigment in blood, or droplets of fat in milk – hold some startling surprises. An experiment carried out 15 years ago showed that soft particles made of polymers – so-called microgels –shrink abruptly when their concentration in a solvent is increased above a certain threshold. When this happens, large particles contract until they are the size of their smaller neighbours. Amazingly, this happens even when the particles are not actually in contact with each other.

Hypothesis of 2016 confirmed

The physicist has been studying the miraculous shrinking of microgels in colloids for the past ten years. In 2016 they explain the phenomenon. The polymer particles consist of long carbon chains. These carry a weak negative charge at one end. These chains form a ball, the microgel. This can be thought of as resembling a ball of wool, with the properties of a sponge. This three-dimensional tangle therefore contains negative point charges that attract positively charged ions in the liquid. These so-called counterions arrange themselves around the negative charges in the ball, forming a positively charged cloud on the surface of the microgel. When the microgels come close together, their charge clouds overlap. This in turn increases the pressure inside the liquid, which compresses the microgel particles until a new equilibrium is reached.

The team formed by researchers Alberto Fernandez-Nieves from UBICS and the Faculty of Physics of the University of Barcelona, and Urs Gasser together with his PhD student Boyang Zhou, from the PSI has now furnished the evidence that supports the 2016 hypothesis. The results have been published in the journal Nature Communications.

Applications in cosmetics and pharmaceuticals

Knowing how soft microgels behave in colloids means that they can be tailored to fit many different applications. In the oil industry, they are pumped into underground reservoirs to adjust the viscosity of the oil in the well and facilitate its extraction. In cosmetics, they give creams the desired consistency. Smart microgels are also conceivable, which could be loaded with medicines. The particles could react to gastric acid, for example, and release the drug by shrinking. Or else a microgel could shrink into a small, densely packed polymer ball when the temperature increases, one that reflects light differently than in its swollen state. This could be used as a temperature sensor in narrow fluid channels. Other sensors could be designed to respond to changes in pressure or contamination.

For further information: https://www.psi.ch/en/media/our-research/mystery-of-microgels-solved